A large number of the electrolytic cells now in existence employ diaphragms. Many of the cells used in the production of chlorine and caustic by the electrolysis of brine are diaphragm type cells. In general, these diaphragms are formed by deposition directly on a foraminous cathode. In the past, the deposition could be expected to be from a slurry of asbestos fibers. Such asbestos diaphragms have the serious disadvantage that in operation the asbestos swells considerably, e.g., up to 800 percent, filling the anode-cathode gap and thus increasing cell voltage and subjecting the diaphragm itself to attrition by gas released at the now proximate anode surface.
One variation to alleviate swelling of a conventional asbestos diaphragm for electrolytic cells for chlorine and caustic production was to make the diaphragm from asbestos together with a polymer, particularly a fluorine-containing polymer. See for instance, (Fenn et al) U.S. Pat. No. 4,410,411 and (Motani et al) U.S. Pat. No. 4,070,257. Related art, (Leduc et al) U.S. Pat. Nos. 3,723,264 and 3,694,281 disclose asbestos-polymer diaphagms for electrolytic cells for olefin production.
Also, some background art shows zirconia-modified asbestos diaphragms, such as (Hruska et al) U.S. Pat. No. 4,354,900. In this patent zirconia is added as a solution or powder merely as a modifier of the asbestos fiber and thus the diaphragm is not made of a composite fiber, but rather a mixture of the asbestos fiber and polymer modifier.
It has also been proposed to incorporate extremely finely divided particles into molten thermoplastics. The resulting thermoplastics can then be shaped. For example, U S. Pat. No. 4,126,536 discusses the addition of sub-micron-sized particles, such as of titanium dioxide to such a molten substance. In shaping, the resultant material can be extruded through a die and the extrudate broken into fibers useful for preparing diaphragms. However, the resulting product has the particles incorporated in the fiber, i.e., encapsulated therein; thereby losing the surface character of the fine particles within the polymer.
Lastly, general background art includes a modification of a Teflon.TM. felt diaphragm with zirconia/magnesia for use in an electrolytic cell for chlorine and alkali metal hydroxide production. This is represented by U.S. Pat. No. 4,253,935. But like other background art, the patent to Simmons does not make any composite fiber diaphragm, but rather uses an inorganic to coat the polymer.
However, the polymer-modified asbestos diaphragms known in the art still show around 25% swelling of the asbestos, and thus are still subject to some chemcial attack and are not substantially resistant to changes in operating conditions. Therefore, they have a limited life.
In all of this prior art of polymer and inorganic, the mixture of polymer and inorganic in the diaphragm does not exhibit any non-uniform morphology in the fiber product structure. According to the prior art, the concurrent use of a polymer and an inorganic in a diaphragm does not require the formation of any non-isotropic intermediate, composite fiber in making the diaphragm. In other words, a composite fiber is not formed, but rather the polymer is simply used to "glue" the inorganic fiber together in the diaphragm.